DNA Replication and Protein Synthesis. DNA Replication

DNA Replication and Protein Synthesis

DNA Replication

DNA replication and protein synthesis happens mainly during S phase.

How DNA Replication Occurs

• DNA replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission

• One copy moves to each new cell

• Since the two strands are complementary, each strandserves as a template to make anew strand

Steps of DNA Replication

1. Helicases separate the DNA Strands- These move along the DNA molecule breaking

the hydrogen bonds between the complementary bases

- This allows the two strands to separate

- The Y shaped region where the strands are separating is called the replication fork (not spoon)

Steps of DNA Replication

2. DNA polymerases add complementary nucleotides to each of the original strands- Nucleotides float freely in the nucleus- As nucleotides are added, bonds form

between the sugar of one nucleotide and the phosphate group of the next

- Hydrogen bonds form between the bases

Steps of DNA Replication

3. DNA polymerases are finished and fall off- Result is two molecules of DNA identical to the original- Molecules are ready to move into new cells during division- Called semi-conservative replication because each of the new DNA molecules has

kept (or conserved) one of the two (or semi) original DNA strands.

Action at the Replication Fork

• DNA synthesis occurs in different directions on each strand

• As the replication fork moves along the original DNA, synthesis follows the movement of the replication fork

• Synthesis on the other strand moves in the opposite direction, away from the replication fork

• This leaves gaps in the newly synthesized strand, which are later joined by an enzyme DNA ligase

Prokaryotic and Eukaryotic Replication

Prokaryotic•Circular chromosome•Replication begins at one place

along the chromosomes•Two replication forks are formed

and proceed in opposite directions•Replication continues along each

fork until they meet an the whole molecule is copied

Prokaryotic and Eukaryotic Replication

Eukaryotic•Each chromosome is long, but not circular•At the rate a polymerase adds nucleotides (50/sec),

it would take 53 days to replicate the largest human chromosome• Instead, replication begins at many points or origins

along the DNA, and two forks move in opposite directions• In a fruit fly, replication begins simultaneously at

about 3500 sites in a DNA molecule

Errors in DNA Replication

• DNA replication usually occurs with great accuracy (only one error for every billion nucleotides added)• How is this done??? • Polymerases have repair or “proofreading”

functions in the same way the computer checks for spelling errors• For example, if A is paired with C, the

polymerase removes the C and replaces it with T

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Errors in DNA Replication

• When mistakes do occur, the base sequence of the newly formed DNA differs from the original• A change in the nucleotide sequence

of a DNA molecule is called a mutation• These can have serious effects on

the function of a gene – therefore disrupting cell function

DNA Replication and Cancer

• Mutations can arise and lead to altered cells and organisms• The changes allow individuals to

survive and reproduce better• Sometimes un-repaired

mutations can cause cancer• Tumors are caused by mutations

that affect cell division

How is the exact replication of DNA

ensured?By complementary base pairing and proofreading by DNA polymerases

What are replication forks?

Areas of DNA where the double helix separates prior to replication

Protein Synthesis

Three Basic Steps to Protein Synthesis• Transcription-DNA code is copied by mRNA (messenger RNA). Happens

in nucleus.• Translation-Ribosomes use information from mRNA to produce

proteins. Happens in cytoplasm.• Protein Synthesis- ribosomes make proteins with tRNA anticodons

Transcription Decoded

• During transcription, RNA polymerase (enzyme) binds to DNA and splits the DNA strands.

• RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of mRNA.

•How does RNA polymerase “know” where to start and stop making an RNA copy of DNA?

Promoters

• RNA polymerase will bind only to regions of the DNA known as promoters, which have specific base sequences.

• Base sequences AUG means “start” in DNA codon language

•Transcription video clip now!!!

The genetic code

• Proteins are made by joining long chains of amino acids called polypeptides (proteins).• Each polypeptide contains a combination of 20 amino acids.

Genetic Code

• The genetic code is read three letters at a time (a codon), so that each “word” of coded messages is three bases long.

• Example • UCG-CAC-GGU

• Stands for serine- histidine - glycine

Translation & Protein Synthesis

• mRNA code is copied by tRNA in sets of 3 bases at a time (codons). • Protein Synthesis-tRNA delivers amino acids to ribosomes which link

amino acids together to make a protein.

tRNAamino acid

attachment site

anticodon

U A C

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